American comedian Jerry Lewis has a message for concerned fans after he was forced to cancel the Sydney leg of his Australian tour: "I'm not pregnant".

Joking aside, Muscular Dystrophy Foundation Australia CEO David Jack says the Hollywood veteran is very disappointed he had to cancel a sold-out show at Rooty Hill RSL on Friday night due to exhaustion.

Lewis arrived in Australia last Monday for a national tour to raise money for the foundation, of which he is an international patron.

But the culmination of a long flight from Las Vegas and a very busy schedule left him feeling "exhausted and unable to perform" to the 700-strong crowd at Rooty Hill on Friday, Mr Jack said.

"When Jerry lands he goes at it 110 per cent," he said.

The 85-year-old performed at a gala dinner for 250 people in Brisbane on Wednesday night, before travelling to Sydney where he began to feel unwell.

On Friday, he was treated at his Sydney hotel by a doctor for symptoms including listlessness and low energy.

As a result, the rest of the Sydney leg of his tour has been cancelled, including a dinner for 300 people at Doltone House in Pyrmont on Saturday night and a lunch appearance at Cronulla on Sunday.

Lewis was feeling a lot better on Saturday afternoon, but would continue to rest before flying to Melbourne on Wednesday where he is due to perform to 1,300 people at Docklands on Thursday, Mr Jack said.

Whether that show goes ahead will depend on his condition.

"Understandably, Jerry's health is our primary concern throughout this period and we want to ensure he's able to travel home healthy," Mr Jack said.

"That's very sad for Jerry because he's so passionate and committed to Muscular Dystrophy."

Mr Jack said he had spoken to Lewis earlier on Saturday, and the comedian was in good spirits.

"He wanted me to give a very important message just to quash any rumours: 'I'm not pregnant'."

But the star was very disappointed at being forced to call off the Rooty Hill show, which was only his third cancellation in a lifetime of performing, Mr Jack said.

"Jerry wanted us to convey a very sincere apology on his behalf to the fans," he said, adding the star was grateful for the supportive messages he had received from fans.

"We were overwhelmed by the positive reaction from people who attended last night's show - they gave Jerry a standing ovation when it was announced he was unable to take the stage and many have elected to donate their ticket costs to the charity rather than seek a refund."

Lewis has been the national chairman of Muscular Dystrophy Association in the US for more than 60 years and has raised more than $2.5 billion for the charity.

He was due to leave Australia next Sunday, but will now fly out a day early, Mr Jack said.